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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_asio.reference.require"></a><a class="link" href="require.html" title="require">require</a>
</h3></div></div></div>
<p>
        <a class="indexterm" name="boost_asio.indexterm.require"></a> 
A customisation point that applies a concept-preserving
        property to an object.
      </p>
<pre class="programlisting">constexpr unspecified require = unspecified;
</pre>
<p>
        The name <code class="computeroutput">require</code> denotes a customisation point object. The expression
        <code class="computeroutput">boost::asio::require(E, P0, Pn...)</code> for some subexpressions
        <code class="computeroutput">E</code> and <code class="computeroutput">P0</code>, and where <code class="computeroutput">Pn...</code> represents
        <code class="computeroutput">N</code> subexpressions (where <code class="computeroutput">N</code> is 0 or more, and with
        types <code class="computeroutput">T = decay_t&lt;decltype(E)&gt;</code> and <code class="computeroutput">Prop0 = decay_t&lt;decltype(P0)&gt;</code>)
        is expression-equivalent to:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            If <code class="computeroutput">is_applicable_property_v&lt;T, Prop0&gt; &amp;&amp; Prop0::is_requirable</code>
            is not a well-formed constant expression with value <code class="computeroutput">true</code>,
            <code class="computeroutput">boost::asio::require(E, P0, Pn...)</code> is ill-formed.
          </li>
<li class="listitem">
            Otherwise, <code class="computeroutput">E</code> if <code class="computeroutput">N == 0</code> and the expression
            <code class="computeroutput">Prop0::template static_query_v&lt;T&gt; == Prop0::value()</code>
            is a well-formed constant expression with value <code class="computeroutput">true</code>.
          </li>
<li class="listitem">
            Otherwise, <code class="computeroutput">(E).require(P0)</code> if <code class="computeroutput">N == 0</code> and the
            expression <code class="computeroutput">(E).require(P0)</code> is a valid expression.
          </li>
<li class="listitem">
            Otherwise, <code class="computeroutput">require(E, P0)</code> if <code class="computeroutput">N == 0</code> and the
            expression <code class="computeroutput">require(E, P0)</code> is a valid expression with overload
            resolution performed in a context that does not include the declaration
            of the <code class="computeroutput">require</code> customization point object.
          </li>
<li class="listitem">
            Otherwise, <code class="computeroutput">boost::asio::require(boost::asio::require(E, P0), Pn...)</code>
            if <code class="computeroutput">N &gt; 0</code> and the expression <code class="computeroutput">boost::asio::require(boost::asio::require(E,
            P0), Pn...)</code> is a valid expression.
          </li>
<li class="listitem">
            Otherwise, <code class="computeroutput">boost::asio::require(E, P0, Pn...)</code> is ill-formed.
          </li>
</ul></div>
<h5>
<a name="boost_asio.reference.require.h0"></a>
        <span class="phrase"><a name="boost_asio.reference.require.requirements"></a></span><a class="link" href="require.html#boost_asio.reference.require.requirements">Requirements</a>
      </h5>
<p>
        <span class="emphasis"><em>Header: </em></span><code class="literal">boost/asio/require.hpp</code>
      </p>
<p>
        <span class="emphasis"><em>Convenience header: </em></span><code class="literal">boost/asio.hpp</code>
      </p>
</div>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2021 Christopher
      M. Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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